X-ray tube window



. 22, 1953 N. c. JAMISON ET AL 2,663,312

X-RAY TUBE WINDOW Filed March 4, 1950 NOEL G QJAMISON ZHOMAS R. K OHLER INVENTORS.

AGENI.

i atentecl Dec. 22, 1951 X-RAY TUBE WINDOW Noel C. Jamison and Thomas R. Kohler, Irvington-on-Hudson, N. Y., assignors to Philips Laboratories, Inc., Irvington-on-Hudson, N. Y.

Application March 4, 1950, Serial No. 147,644

, This invention relates to X-ray tubes, particularly for X-ray diffraction study.

X-ray tubes for diffraction study require a comparatively large output of relatively pure X-radiation. One of the problems that has been found in connection with tubes for that purpose is to provide a satisfactory window permitting a large output of radiation and yet which is mechanically secure against leakage. It is also desirable in that connection to fabricate and 'assemble'su'ch windows into X-ray tube envelopes inexpensively and without great difficulty. These problems have not been met too satisfactorily in the prior art.

It is an object of our invention to provide an X-ray tube for X-ray diffraction purposes which has a large output of relatively pure 7-radiation It is still another object of our invention to provide a window construction for X-ray diffraction tube which is simple and inexpensive to fabricate and assemble into the tube envelope,

It is a still further object of our invention to provide a window construction for X-ray diffraction tubes which is protected against internal electronic bombardment.

These and further objects of the invention will appear as the specification progresses.

According to the invention, an X-ray tube for diffraction purposes is provided with a window in the form of a thin disc of radiation permeable insulating material sealed into the evacuated envelope in a vacuum-tight manner and provided on its internal surface, 1. e. on the surface facing into the evacuated chamber housing the tube electrodes, with an extremely thin film of metal pervious to X-radiation.

In the preferred embodiment of our invention, the thin disc-like window comprises a mica disc which is coated on its internal surface with an extremely thin film of one of the noble metals. e. g. gold and platinum. We have found that those metals are preferable because ordinary application techniques can be employed since these metals resist oxidation and because they are extremely good electrical conductors. Other metals may be applied, such as beryllium, nickel and aluminum, in which case the window must be assembled into a tube in such manner as to avoid any oxidation or contamination of the metal.

In applying the metal films, any conventional deposition technique may be employed such as by vacuum evaporation, or sputtering. In this manner, films of the order of thickness of hundredths of a micron are laid down on the mica 7 Claims. (Cl. 313--59) and. appear transparent to light. Measurements have shown that the surface resistivity of such films varies, depending upon the thickness of the deposit, between about 10 and 1000 ohms.

' In fabricating the window, a mica disc is placed in a metal sealing ring serving as a support for the window, and at several points around the periphery of the window where the latter joins the sealing ring, small amounts of silver paste are applied and the assembly heated in a furnace to a temperature of about 300 to 350 C. to form a conductive joint between the window face and the ring. The metallic film is then applied to the mica disc, i. e. by vacuum evaporation or sputtering. Finally, the assembly is sealed into the envelope of the tube.

The window as constructed, is highly permeable to X-radiation, i. e., its absorption of radiation has been found to be about 1 to 2.5% for CuKa radiation, and resists cracking due to internal stresses probably resulting from internal electronic bombardment caused by secondary emission from the anode target. It is highly probable that secondary emission results in internal voltage stresses on the internal surface of the mica disc in the absence of the thin conducting film which results in a rupture of the mica dielectric and consequent failure of the window. The thin metallic film, however, serves to maintain the window at substantially the same potential as the other parts of the envelope, thus easing or eliminating voltage stresses on the window.

The invention will be described with reference to the drawing in which:

Fig. 1 shows an X-ray diffraction with a mica window; and

Fig. 2 shows the details of the window construction.

Fig. 1 shows an X-ray tube used in X-ray diffraction study which comprises an evacuated envelope I mounted on a base 2 and provided with an anode 3 and a cathode 4 and four windows 5 for viewing the anode target from four separate positions. Power connections for the anode and cathode are provided in the base.

The details of the window are shown in Fig. 2 which comprise a metal sealing ring 6 vacuum sealed to the envelope. Within the sealing ring and mounted on a shoulder is a mica disc 8 having a thickness of about 0.0003 to 0.0005 inch, which is sealed to the ring with powder glass seals 1, and which is coated on the under side or that side which faces the anode in the completely assembled tube with a very thin film of gold 9 applied by vacuum evaporation. At various points along the periphery of the mica disc or along the entire periphery there is applied a small amount of silver paste I which has been fired to provide a conductive path for the metal film to the sealing ring.

The resulting window construction has excellent transmission characteristics and is protected against undue voltage stress because of the pres.- ence of the thin conducting "film on its inner face. This construction has the further advantage that the anode of the tube is always visible, making it possible to view the anode without breaking the tube seal.

While we have thus described our invention with specific examples and embedimepts til-tree of, other modifications will be readily apparent to those skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.

What we claim is:

1. An X-ray tube comprising an evacuated envelope, cathode and anode electrodes, and a radiation permeable window in said envelope, said window comprising a thin mica disc coated on the internal surface thereof with a thin lighttransparent conductive metallic-film having a surface resistance between and 1000 ohms for protecting the window against internal electronic bombardment.

Y 2. An X-ray tube comprising an evacuated envelope, cathode and anode electrodes, and a radiation permeable window sealed into said en: velope, said window comprising a thin mica disc and a thin light-transparent conductive metallic film of a noble metal having a thickness of I the order of hundredths of a micron and a surface resistance between 10 and 1.000 ohms on the inper surface of said disc toprotect the latter against internal electron bombardment.

3. An X-ray tube comprising an evacuated envelope, cathode and anode electrodes, and a radiation permeable window sealed into said envelope, said window comprising a thin mica disc and a thin light-transparent conductive metallic filmof a metal selected from the grjoup consisting of gold and platinum having a thickness of the order of hundredths of a micron and a surface resistance between 10 and 1000 ohms on the inner surface of said disc to protect the latter againstinternal electron bombardment.

'4. An X-ray tube comprising an evajcuted envelope, cathode and anode electrodes, and a radiation permeable window sealed into said envelope, said window comprising a thin mica disc and a thinlight-transparent 1aver'of'go1d having a thickness of the order of hundredths of a micron and a surface resistance between 10 and 1000 ohms on the inner surface of the mica disc to protect the latter against internal electron bombardment.

5. An X-ray tube comprising an evacuated envelope, cathode and anode electrodes, and a radiation-permeable window vacuum sealed into said envelope, said windowcomprising a metal sealing ring vacuum-sealed to the walls of said envelope, a thin mica disc vacuum-sealed into said sealing ring, and a thin light-transparent layer of gold having a surface resistance between 19 and i000 chins on the inner surface of said mica disc to protect the latter against internal electron-bombardment.

6. An X-ray tube comprising an evacuated envelope-cathode and anode electrodes, and a radiationpermeable window sealed into said envelope, said Window comprising a metal sealing ring vacuum-sealed to the walls of said envelope, 3 thin mica disc powdereglass sealed into said sealing ring, and a thin light-transparent layer of gold having a surface resistance between 10 and 1000 ohms on the inner surface or" said mica disc to protect the latter against internal e13?- tron bombardment.

'7. An X-ray tube comprising an evacuated en velope, cathode and anode electrodes; and a radiation permeable window sealed into said en.- velope, said window comprising a metal sealing ring vacuum-sealed to the walls of said envelope, a thin mica disc powder-glass sealed into said sealing ring, and a thin light-transparent layer of platinum having a surface resistance between 10 and 1000 ohms on the inner surface of said mica .disc to protect the latter againstinternal electron bombardment.

NOEL C. JAMISON. HOMAS R. KOH ER- References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,626,465 Holst et a1. Apr. 26, 1927 1,626,693 Rentschler May 3, 1927 1,679,894 Bucky Aug. 7, 1928 2,009,457 Sloan July'30, 1935 2,148,488 McEuen Feb. 28, "1939 2,205,297 Lenz June 18, 1940 2,390,863 Amidon et a1 Dec. 11, 1945 2,403,079 Higgins July 1945 2,413,604 Colbert et a1. Dec. 31, 19 1.6 

